Add lemon juice to milk. Stir to combine. Set aside.
¾ cup whole milk, 2 teaspoons lemon juice
Add rolled oats to a blender or food processor. Blend or process until oats are ground into a flour. Add to a large bowl.
1½ cup rolled oats
Whisk in cocoa powder, baking powder, and salt. Set aside.
¼ cup unsweetened cocoa powder, 3 teaspoons baking powder, ¼ teaspoon salt
Add milk, yogurt, egg yolks, sugar, vanilla extract, and butter to a blender or medium bowl. Blend or mix with an electric mixer on low speed until frothy.
⅓ cup granulated sugar, 1 cup Greek yogurt, 2 large eggs, 1 teaspoon vanilla extract, 2 teaspoons unsalted butter
Pour wet ingredients into dry ingredients and stir until no dry ingredient streaks remain. Stir in egg whites until incorporated. Set aside to rest at least five minutes.
2 large eggs
In the meantime, heat a griddle or large skillet to medium heat. Melt some butter over the hot surface and add about ¼ cup of batter to the surface. Repeat until there is no more room on the cooking surface. Cook pancakes until bubbles form and pop on the surface, about 4-5 minutes. Then flip and cook about 4 minutes longer. Remove pancakes to a plate or baking sheet. Repeat this step until the batter is gone.